[media] lirc_imon: do not leave imon_probe() with mutex held

Commit af8a819a ("[media] lirc_imon: simplify error handling code")
lost mutex_unlock(&context->ctx_lock), so imon_probe() exits with
the context->ctx_lock mutex acquired.

The patch adds mutex_unlock(&context->ctx_lock) back.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: af8a819a ("[media] lirc_imon: simplify error handling code")

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>